1. Field of the Invention
The present invention relates to the automotive field. More specifically, the invention relates to a method of detecting the motion of a vehicle and to a corresponding device, used in conjunction with a system for monitoring tire pressure.
2. Description of the Related Art
Conventionally, in order to monitor vehicle wheel tire pressures, each wheel is equipped with a wheel unit. This wheel unit is, for example, attached to the valve of the tire and is held firmly against the rim on the inside of the tire.
In a known way, each wheel unit comprises at least one pressure sensor capable of monitoring the pressure obtained inside the tire. The information relating to the pressure is then sent, via a wireless communications means, to a central processing unit situated inside the vehicle. The pressure measurements are analyzed and processed by this central processing unit.
In order that the information from the pressure sensors can be processed at a higher frequency when the vehicle is in motion, it is known practice for the vehicles to be fitted with a motion sensor. This is because rapid processing of defective-pressure information is of key importance if the vehicle is in motion. When the vehicle is stationary, this information do not need to be processed quite so urgently. It is commonplace for information from the tire pressure monitoring system to be processed at two different paces according to whether or not the vehicle is in motion.
In order to determine whether the vehicle is moving or stationary it is commonplace for at least one wheel unit to be equipped with a vehicle motion detection means. Advantageously, the information obtained by the motion detection means mounted in the wheel is transmitted by the very same wireless communications means as is used for the tire pressure. This information is analyzed by the central processing unit.
It is also known practice for the vehicle to be equipped with a motion detection means independent of the wheel unit. However, that solution is of little advantage in terms of cost.
One known motion detection means consists, for example, of an accelerometer mounted in the wheel unit. Indeed the acceleration to which a wheel is subjected is directly dependent on whether (or not) this wheel is rotating. However, accelerometers are highly sensitive to temperature variations and, as a result, temperature compensation has to be provided in order to correct the accelerometer measurements, making accelerometers expensive and complicated to use.
In order to detect quickly the onset of vehicle motion, the central processing unit regularly monitors the information from the wheel unit. This monitoring and the processing of the corresponding data are carried out approximately every 10 seconds whether or not the vehicle is stationary. As a result, there is a risk that the vehicle battery will be run down prematurely if the vehicle remains stationary for a long period. The use of an accelerometer by way of a vehicle motion detection means therefore proves to be a means that is expensive and not very economical with energy.
In order to detect vehicle motion it is also known practice for at least one wheel unit to be equipped with a sensor that senses the earth's magnetic field, such as an electromagnetic coil for example, these being particularly inexpensive. What happens is that when the vehicle is stationary, the earth's magnetic field as measured by the wheel unit is substantially constant. By contrast, when the vehicle is in motion, the wheels are rotated and the magnetic sensor situated in the wheel detects variations in the earth's magnetic field. It is thus easy to determine whether or not the vehicle is in motion simply by monitoring whether or not the magnetic-field sensor is detecting variations in the magnetic field.
Unfortunately, there are numerous parasitic signals that may be interpreted by the wheel unit as an indication that the vehicle is in motion. Specifically, when a vehicle is stationary near to something that generates parasitic signals, the magnetic sensor may be sensitive to these parasitic signals and send the wheel unit microprocessor a signal (believing it to be a measurement indicating that the vehicle is actually in motion) even though in fact it is merely a disruption to the magnetic field due to a parasitic generator. When this happens, the central processing unit mounted in the vehicle has to process this parasitic signal. The best possible outcome is that, having processed it, it may realize that this was a parasitic signal and disregard it. However, it may just as easily not realize it and send incorrect information to the driver. In both instances, it is a needless expenditure of energy for processing the parasitic signal, which thus carries the risk of running down the vehicle battery.
Items that generate parasitic signals, in respect of magnetic sensors, are in fact relatively commonplace and are all the more disruptive when the vehicle is stationary. What happens in such circumstances is that the generator continuously disrupts the motion detection means and the vehicle central processing unit is constantly and needlessly having to process signals that have nothing to do with vehicle motion. Generators such as this may, for example, be a washing machine or a refrigerator positioned in a garage beside the vehicle. They may also be electromagnetic waves due to the proximity of an electric socket (220 V at 50 Hz or at 60 Hz). The vehicle may also be disturbed when stationary if parked near an electrical transformer, near a high tension cable, near a tramway, etc. In the case of a tramway for example, the magnetic field induced is a field that varies at about 16 Hz. A field such as this may be interpreted, incorrectly, as being associated with the vehicle moving at a speed of 110 km/h.